The merozoite surface protein-1 (MSP-1) has become the most significant single candidate for a vaccine against the erythrocytic stages of malaria. Despite long-standing observations that parasite-derived MSP-1 protein could elicit protective host responses in both primate and rodent models of malaria, there has been limited information on the regions of the molecule required for protection and the mechanisms involved in this immunity. Our laboratory has been studying the MSP-1 protein from the rodent malarial parasite P. yoelii based on our earlier identification of a monoclonal antibody which had dramatic protective activity against a normally lethal challenge infection. We subsequently established that this antibody recognizes a discontinuous, disulfide-dependent epitope in the conserved, carboxyl-terminal region of the MSP-1 protein. Others have suggested that the cysteines in this region are arranged in two epidermal growth factor (EGF)-like domalns and have shown that it is retained on the merozoite surface after erythrocyte invasion. ObtaIning recombinant polypeptide which mimicked the structure of the carboxyl-terminal region of MSP-1 has proven to be difficult, presumably due to the requirement for proper disulfide pairing. However, in the previous granting period we have shown that the two EGF-like domains of the P. yoelli MSP-1 protein can be expressed in a native configuration when synthesized as a fusion protein with glutathione-S-transferase (GST). Moreover, we have shown that mice immunized with this antigen containing only 10kDa of plasmodial sequence can be almost completely protected against a normally lethal challenge infection. Our experiments to date support the role of serum antibodies in this protection. These findings have provided a model system for studying the immune response to a major erythrocytic-stage antigen of malaria parasites. We now propose a series of studies to investigate the variables surrounding the induction of protective immunity, to analyze in detail the serological responses induced with an emphasis on identification of protective antibodies, to examine the T-cell responses to epitopes derived from the MSP-1 molecule, and to begin to explore the structure and function of the carboxyl-terminal region of the protein. These findings may provide information relevant to the development of a successful vaccine against erythrocytic-stage infection and also illuminate the biological significance of the MSP-1 protein in the plasmodial life cycle.